Keeper Ines hands the crew the Saturday door count from week 4. It lists how many people came through the door each hour.
"Let's make another bar graph," Comet says.
Wren runs a finger down the hours. "What do you notice? These numbers rise and fall through the day."
"So we want to see change over time," Comet says slowly.
Then she spots the bike rack out the window. "I bet more bikes show up on warm days. Can a graph show that too?"
Nova hovers between them. "Would you like a hint?" she asks.
"One kind of graph follows a single thing over time. Another kind puts two things side by side to see if they move together."
Line graphs track changes over short and long periods of time.
When the changes are small, line graphs are better to use than bar graphs.
A line graph can also compare changes over the same period for more than one group.
| Hour (Saturday) | People through the door |
|---|---|
| 10 am | 5 |
| 11 am | 8 |
| 12 pm | 6 |
| 1 pm | 7 |
| 2 pm | 9 |
| 3 pm | 11 |
| 4 pm | 14 |
| 5 pm | 16 |
| 6 pm | 12 |
| 7 pm | 7 |
| Total | 95 |
Made-up data from the Commons Count story.
An X-Y plot, or scatter plot, is used to find a relationship between two different things.
If both variables increase at the same time, they have a positive relationship.
If one variable decreases while the other increases, they have a negative relationship.
Sometimes the variables do not follow any pattern. Then they have no relationship.
| Made-up day | Noon temperature (degrees) | Bikes in the rack | Rainy hours | Date in the month |
|---|---|---|---|---|
| Day A | 50 | 2 | 6 | 9 |
| Day B | 58 | 4 | 4 | 21 |
| Day C | 64 | 5 | 2 | 3 |
| Day D | 70 | 7 | 1 | 28 |
| Day E | 77 | 9 | 0 | 14 |
This bike rack data is made up for the lesson.
A variable is anything you try to measure, such as an object, an event or a time period.
An independent variable stands alone. It is not changed by the other variables you measure.
A dependent variable depends on other factors.
Bikes in the rack cannot change the temperature, so temperature is the independent variable here.
| Statement | True or false? |
|---|---|
| Line graphs can track changes over short and long periods. | ? |
| Line graphs are better than bar graphs when the changes are small. | ? |
| An X-Y plot can show that two things have no relationship. | ? |
| In a negative relationship, both variables go up together. | ? |
Sharp work, data analyst. Tomorrow is Impact Friday: what happens when a graph leaves the workbench.